The present invention relates to a method and an apparatus for tightening bolts. In particular, it relates to those that is suitably applicable to a bolt having bolt tightening torque properties, in which a torque gradient expressed in terms of the ratio of an additional tightening torque exerted on a bolt to its additional turning angle has a first torque gradient in an initial bolt tightening stage and a second torque gradient in a late bolt tightening stage subsequent to the initial bolt tightening stage, a magnitude of the second torque gradient being greater than that of the first torque gradient.
A bolt tightening method in which at first the bolt is tightening until a bolt tightening torque reaches (becomes) a predetermined snug torque and subsequently the bolt is turned by a predetermined initial-set turning angle (hereinafter, referred to as “torque+angle method”) is well known.
This conventional torque+angle method (snug torque TA+initial-set turning angle θA) provides a relatively stable bolt axial force if relationships between the bolt turning angle θ and the bolt tightening torque T were substantially proportional, for example, as shown by tightening torque properties C1 in FIG. 10. However, when the conventional torque+angle method is applied to a case where the properties have bent properties, like C2 or C2′ in the figure, comprising an initial bolt tightening stage I having a smaller torque gradient and a late bolt tightening stage II having a larger torque (torque gradient being expressed in terms of the ratio of an additional tightening torque exerted on the bolt to its additional turning angle), there is a problem that the bolt axial force would scatter (become unstable) according to scattering of bent position of the properties.
The above-described properties like the property C2 appear in a case, for example, where an elastic deforming member is located between fastened members and these fastened members are fastened with the elastic deforming member, in which the elastic deforming member would be eliminated (crushed) in the above-described initial bolt tightening stage I.
Meanwhile, in a case where the fastened member are fastened with no elastic deforming member between them or the mount of the above-described elimination (crush) is almost zero, the properties would be like C1. If the elimination has a certain degree of amount, it would be like the properties C2, and if the amount of elimination is more than that, it would be like the properties C2′. Accordingly, since the bent position scatters according to changing of the amount of this elimination as shown in FIG. 10, the bolt axial force also scatters.
Further, part of the bolt axial force is consumed (used) for the elimination of the elastic deforming member in the initial bolt tightening stage I of the properties C2, C2′. As a result, there is a problem that the effective bolt axial force acting on the fastened members would decrease eventually.
Another method (hereinafter referred to as “seating-point angle method”) that can solve the above-described problems, which is shown in FIG. 11, is also known (for example, see Japanese Patent Laid-Open Publication No. 2-41830).
FIG. 11 is a graph showing the seating-point angle method. Herein, in the case of the bolt tightening torque properties C1 (the same as C1 of FIG. 10), the bolt is turned from a tightening start point (the bolt turning angle θ=0) to an initial-set turning angle θ91 (point P91).
In the case of the bent properties C2, C2′, meanwhile, at first the bolt is turned by the initial-set turning angle θ91 from respective theoretical seating points P93 (the bolt turning angle θ=θ93), P95 (the bolt turning angle θ=θ95) and subsequently it is further turned by respective additional-turning angle θC, θC′. Thus, the bolt tightening is finished (at points P97, P98).
Since the seating-point angle method is described in detail in the above-described publication, hereinafter the features of the method will be described briefly. The first feature of that is that the starting point of the bolt turning angle is shifted from its actual starting point (the bolt turning angle θ=0) to the theoretical seating points P93 (the bolt turning angle θ=θ93), P95 (the bolt turning angle θ=θ95) in the late bolt tightening stage II. The second feature is that the bolt is further turned by a specified angle (the corrective additional-turning angle θC, θC′) that is equivalent to the decrease amount of the effective bolt axial force acting on the fastened members due to the existence of the initial bolt tightening stage I (the crush of the elastic deforming member in the above-described example).
The theoretical seating point P93, which is a hypothetical point as a bolt tightening starting position when the bolt torque properties C2 is assumed to be substantially linear like the bolt torque properties C1, is defined as a point of intersection of an extension line of the bolt tightening torque properties C2 in the late tightening stage II and the line of the bolt tightening torque T=0. When the bent position of the bolt tightening torque properties C2 scatters and changes to the properties like the C2′, the theoretical seating point P93 shifts to the point P95 (the bolt turning angle θ=θ95) as shown in the figure. Because the theoretical seating point shifts accordingly even if the bent position of the properties scatters, an influence caused by this scatter of the bent position can be properly suppressed by setting the bolt turning angle to the theoretical seating point.
The corrective additional-turning angle θC of the second feature of the seating-point angle method is defined as an angle difference between the theoretical seating point P93 and the bent position P94 (the intersection point of the properties in the initial bolt tightening stage I and the properties in late bolt tightening stage II). The bolt tightening torque T94 corresponding to the bent position P94 is a bolt tightening torque that is equivalent to the decrease amount of the effective bolt axial force acting on the fastened members due to the existence of the initial bolt tightening stage I. Accordingly, this decrease of the effective bolt axial force can be compensated (corrected) by turning the bolt additionally by the angle θC corresponding to the bolt tightening torque T94 in the late bolt tightening stage II.
When the bent position of the bolt tightening torque properties C2 scatters and changes to the properties like the C2′, the bent position P94 shifts to the bent point P96 (the bolt tightening torque T=T96) as shown in the figure. T96 is greater than T94, which is influenced by the initial bolt tightening stage I. Meanwhile, the corrective additional-turning angle θC changes to θC′, which is increased from θC by a specified amount of angle that is proportional to the decrease amount of the effective bolt axial force acting on the fastened members.
Namely, by setting the corrective additional-turning angle θC this way, the decrease of the effective bolt axial force acting on the fastened member due to the existence of the initial bolt tightening stage I can be effectively compensated, and the proper additional-turning angle can be obtained regardless of a location of the bent position P94 of the bolt tightening torque properties C2.
Thus, according to the seating-point angle method, the bolt is turned until the bolt turning angle θ reaches a total angle of the bolt turning angle θ93 corresponding to the theoretical seating point P93, the initial-set turning angle θ91, and the corrective additional-turning angle θC. Then, the bolt tightening is finished at this point (the point P97). In the case of the properties C2′, the bolt is tuned until it reaches the point P98 as shown in the figure, where the bolt tightening is finished.
In both cases, the scatter of the bolt axial force due to the change of bent position can be suppressed eventually and the decrease of the effective bolt axial force acting on the fastened members due to the existence of the initial bolt tightening stage I can be compensated (corrected) properly as well.
As described above, according to the seating-point angle method disclosed in the above publication, the scatter of the bolt axial force due to the change of bent position can be suppressed and the decrease of the effective bolt axial force acting on the fastened members due to the existence of the initial bolt tightening stage I can be compensated properly even for the bolt having the bent tightening torque properties described above. However, most of the facilities or apparatuses that prevail nowadays are designed for the torque+angle method. Since the torque+angle method is considerably different from the seating-point angle method, it may not be so easy to apply the seating-point angle method to such facilities or apparatuses. Namely, the starting point of the bolt turning angle θ in the torque+angle method is a reaching point of the snug torque TA (the point P3 in FIG. 10) and the bolt turning-angle control starts from this point. In the seating-point angle method shown in FIG. 11, however, the bolt turning-angle control is required from the initial stage of the bolt turning. Thus, the control manners are considerably different from each other.
Accordingly, a new bolt tightening method has been desired, in which the torque+angle method is adopted basically so as not to need to remake the conventional facilities or apparatus greatly and the scatter of the bolt axial force due to the change of bent position can be suppressed and the decrease of the effective bolt axial force acting on the fastened members due to the existence of the initial bolt tightening stage I can be compensated properly even for the bolt having the bent tightening torque properties.